In drilled wells, a steel casing is set within a wellbore formation to maintain the structural integrity of the entire well and to prevent the wellbore walls from collapsing inward. An annular region defined by the space between the steel casing and the wellbore formation is commonly filled with cement to stabilize the steel casing and to prevent fluid communication between vertically stratified geological zones. This cement must be inspected to confirm its structural integrity and to assure a complete sealing of the annulus between the steel casing and wellbore formation.
Imaging tools which are lowered into the open hole utilizing an electric wireline or cable typically construct a graphic representation of acoustic reflection properties and calculate acoustic travel times from the tool exterior to the wellbore wall. The graphic representation approximates a visual image of the wellbore wall.
Steel has a greater acoustic impedance than the impedances of fluids disposed between the imaging tool and the steel casing. Steel also has a greater acoustic impedance than the cement disposed outside the steel casing. As a result, reflective reflections from inner and outer surfaces of the steel casing frustrate attempts to image areas within the cement located an inch or more behind casing. More specifically, multiple acoustic reflections in the wellbore fluid and within the casing occur simultaneously with the arrival of acoustic reflections from the cement onto the acoustic sensors. Furthermore, strong impedance mismatches at the wellbore fluid-steel and steel-cement boundaries greatly decrease energy propagating into the cement and returning from reflectors in cement and back to the acoustic sensors, therefore increasing the difficulty of imaging the concrete. In drilled wells, steel casing is installed to maintain integrity and to prevent walls from caving. The annular region between casing and formation is filled with cement to prevent fluid communication between different geological zones. This cement must be inspected to confirm complete sealing of the annulus between casing and formation.
Steel has large acoustic impedance compared of fluid inside casing and cement outside casing. The resulting reverberation of energy trapped in casing makes it difficult to image reflectors located behind casing. Multiple reverberations in casing fluid and in casing occur simultaneously with the arrival of echoes from certain cement reflectors. Additionally, strong impedance mismatches at steel boundaries cause echoes from reflectors in cement to be much smaller than reverberation noise.